Are Greenhouse Gases Upping the
Risks of Flooding, Too?

Climate change caused by rising concentrations of
greenhouse gases in the atmosphere is causing more
extreme rainfall and snowfall--and floods

Feb 27, 2011 - scientificamerican.com

In autumn 2000 devastating floods swept through
England and Wales, inundating homes, swamping roads
and rail lines, and requiring the evacuation of more
than 11,000 people. That fall was the wettest in
the region since records began in 1766, and the subsequent
flooding caused billions of dollars in damage. Now
climatologists suggest that climate change doubled
the odds of such catastrophic flooding in 2000.

"Greenhouse gas emissions due to human activity
have affected the odds of floods in England and Wales," says
physicist Pardeep Pall of the Swiss Federal Institute
of Technology (ETH Zurich), who led the research
published February 17 in Nature. "The odds of
a flood occurring in the autumn of 2000 likely increased
by double or more." (Scientific American is
part of the Nature Publishing Group.)

Such inundations are becoming more common, according
to the International Federation of Red Cross and
Red Crescent Societies, among other disaster statistics
keepers. In fact, according to reinsurer Munich Re,
extreme floods have tripled globally since 1980.
The reason may well be climate change caused by increasing
concentrations of atmospheric greenhouse gases—now
roughly 390 parts per million, up from 280 ppm in
the 1700s.

Warmer atmospheric air means more water vapor, which
is itself a greenhouse gas, exacerbating the problem.
What goes up, must come down and, more and more,
that water vapor is coming down in extreme precipitation
events—defined in North America as more than
100 millimeters of rainfall (or the equivalent in
snow or freezing rain) falling in 24 hours—according
to new research also published February 17 in Nature
that examines such events in the Northern Hemisphere.

"We expect a widespread increase in heavy precipitation
due to greenhouse gas warming leading to a moister
atmosphere," explains climatologist Gabriele
Hegerl of the University of Edinburgh in Scotland.
That's because water vapor increases by roughly 7
percent for every degree Celsius of warming in the
lowest level of the atmosphere—or, more simply
put, warmer air means warmer water, which means more
of it in the form of vapor. That vapor still coalesces
into clouds, raindrops and snowflakes, which is why
basic physics suggests that more water vapor results
in more rain. That's exactly what the study found
for the first time. "Our study shows that the
heaviest [precipitation] events have increased in
magnitude, meaning that rare events are becoming
less rare," notes climatologist Francis Zwiers
of the University of Victoria in British Columbia.

Hegerl, Zwiers and their climatologist colleagues
at Environment Canada examined daily records from
more than 6,000 weather stations around the globe
of rainfall, snowfall and other precipitation stretching
from 1951 to 1999. In each year of that period, they
determined how extreme precipitation had been. By
compiling the information from all these years and
comparing it with the precipitation patterns predicted
by computer models of the climate, the scientists
noted a similar pattern emerging in the real-world
data. What's more this pattern could not be explained
by natural climate fluctuations, suggesting that
human-induced climate change is the culprit behind
an increase in downpours and blizzards in the last
50 years of the 20th century—at least in the
Northern Hemisphere.

"There are characteristic patterns of increase
and decrease, for example, in response to an El Nino
event," which is a cyclical climate event marked
by warming waters in the western Pacific Ocean that
has global impacts, Zwiers says. "That's not
the kind of change we saw." Instead a human
fingerprint emerged from the data pattern. More worryingly,
the actual increase in rain, snow and sleet were
larger than predicted by the computer models.

The study marks the first time that human influence
on the climate has been demonstrated in the water
cycle, and outside the bounds of typical physical
responses such as warming deep ocean and sea surface
temperatures or diminishing sea ice and snow cover
extent.

To attribute any specific extreme weather event—such
as the downpours that caused flooding in Pakistan
or Australia, for example—requires running
such computer models thousands of times to detect
any possible human impact amidst all the natural
influences on a given day's weather. "It is
a reasonable question: is human influence anything
to do with this nasty bit of weather we're having?" explains
physicist Myles Allen of the University of Oxford,
who helped oversee the English flooding study. "Answering
it isn't easy."

So, the U.K. team also called on tens of thousands
of volunteers who ran a climate model thousands of
times on their personal computers in the background
as part of the climateprediction.net Web site. After
six years of running such simulations, the verdict
is in: Increasing greenhouse gas concentrations as
a result of burning fossil fuels and cutting down
forests increased the risks of flooding in two out
of three model runs by more than 90 percent.

The bad news is that such record-breaking downpours,
blizzards and sleet storms are likely to continue
to get worse as atmospheric greenhouse gas concentrations
continue to rise, causing global temperatures to
continue to warm and making the atmosphere more and
more humid. "The human influence on the climate
system has the effect of intensifying precipitation
extremes," Zwiers notes. "It is therefore
reasonable to expect that precipitation extremes
will continue to intensify," although how much
is still a mystery, largely thanks to an unclear
understanding of the atmospheric impact of how tiny
flecks of pollution in the atmosphere—known
as aerosols to scientists and comprising materials
ranging from soot to sulfur dioxide.

As for whether the world's recent extreme weather
was made worse by human influence, that answer likely
won't be available for years—and only if a
research effort similar to the one that analyzed
the 2000 U.K. floods is undertaken. "The human
impact in this is still an open question," Zwiers
says.

But the U.K. Met Office (national weather service),
the U.S.'s National Center for Atmospheric Research
and other partners around the globe aim to change
that in the future by developing regular assessments—much
like present evaluations of global average temperatures
along with building from the U.K. flooding risk modeling
efforts—to determine how much a given season's
extreme weather could be attributed to human influence. "We
will develop that science further so that we can
provide regular and scientifically robust evidence
on how the odds of these phenomena are changing," says
climate modeler Peter Stott of the U.K. Met Office.

Already, it is becoming clear that burning fossil
fuels and clearing forests are having an impact on
the atmosphere, which is rebounding to the detriment
of the humans behind those activities. "One
of the problems people find with climate change is
it's a victimless crime…nobody's particularly
affected by small changes in global average temperatures," Allen
notes. "Extreme weather is what actually hurts
people."